1. Field of the Invention
The present invention relates to a garnet polycrystalline film for a magneto-optical recording medium, and more particularly to a garnet polycrystalline film for a magneto-optical recording medium composed of fine crystal grains useful for reduction of a noise derived from a grain boundary of garnet polycrystal.
2. Description of the Related Art
A garnet oxide is a material showing excellent performance as a recording material or a magneto-optical device such as a magneto-optical recording medium, an optical isolator and a magnetic field sensor. Since this material is non-magnetic in an amorphous state in general, it is used in a single crystal or polycrystal state. However, polycrystalline garnet which can be produced inexpensively is inferior in optical, magnetic and magneto-optical characteristics to the single crystal garnet due to existence of a grain boundary. In particular, improvement in performance of polycrystalline garnet is indispensable in order to apply the polycrystalline garnet as a magneto-optical recording medium.
The magneto-optical recording is the most useful technique for realizing high density and high reliability. The garnet material showing high corrosion resistance and having superior magneto-optical effect in a short wavelength is considered to be most promising as a next generation magneto-optical medium. This material is able to overcome defects such as low corrosion resistance and small magneto-optical effect of an amorphous rare earth-transition metal which has been utilized already.
As a method of further improving the recording density, multiple recording with a multilayer film utilizing transparency of garnet has also been proposed as disclosed in the Digests of the 10th Annual Conference on Magnetics in Japan, 31 (1986) by Ito et al.
Further, it has been known that when the garnet film is combined with conventional other metallic magneto-optical recording medium such as amorphous rare earth-transition metal alloy film or a multilayer of Pt and Co or Pd and Co, it is possible to improve magneto-optical effect utilizing the large Faraday rotation angle thereof, which is effective to obtain high performance of a magneto-optical recording medium.
A Bi substituted garnet film formed on a gallium-gadolinium-garnet (GGG) single crystal substrate shows a high performance of 60 dB in a carrier wave to noise ratio (signal to noise ratio under standard conditions) in read/write characteristics (H. Kano et al.: IEEE Trans. Mgn. MAG-25(5), 3737 (1989)). However, a garnet film formed on an inexpensive glass substrate and the like, which is polycrystalline, has such a drawback that a medium noise is big because of optical nonuniformity (nonuniform distribution of refractive index) derived from the grain boundary.
In order to form a polycrystalline garnet film of high performance on a glass substrate and the like, fining of crystal grains is effective to reduce optical nonuniformity due to the grain boundary of crystals (for example, M. Abe and M. Gomi: J. Magn. Magn. Mater;, 84, 222 (1990)). As a method for fining crystal grains, it is known to add specific elements (for example, Ito et al.: The Digests of the 12th Annual Conference on Magnetics in Japan, 127 (1989)) or to apply a rapid (heat treatment) crystallization method (T. Suzuki et al.: The Digests of the 13th Annual Conference on Magnetics in Japan, 49 (1989)). In "Sputtered Garnet Film for Magneto-optical Disk" by Shono. et al., Light and Magnetism--Basis and Application, Magnetics Seminar Text, 107 (1988), Shono has reported that in observation of a garnet film after complete crystallization by a transmission electronic microscope, fine crystal grains are observed in the garnet film formed on a surface of (111) GGG single crystal face having a lattice constant which differs by approximately 1% from that of the garnet film, but no fine crystal grain is observed on a calcium-magnesium-zirconium substituted gadolinium-gallium-garnet (GCGMZ) single crystal substrate having lattice constant which is different by approximately 0.3% of the garnet film and epitaxial growth has been made.
The present inventors have produced amorphous oxide film having a garnet structure (Bi, Ga substituted Dy, Fe garnet) after crystallization, respectively, on GGG single crystal substrates of different orientations, observed a crystallization process by heat treatment in detail, and confirmed that fine garnet crystal grains have been preferentially generated at an interface with the single crystal substrate having a crystal lattice constant which differs by .+-.0.3% or more. This is considered to be caused by a fact that the interface energy with crystalline nucleus generation is less increased at the interface between the amorphous film having a garnet structure after crystallization and the garnet single crystal than inside of the amorphous film or on the film-free surface. A non-uniform nucleus generation occurs with priority at the interface. It is also considered that epitaxial growth is obstructed because of a large misfit of the lattice constants and a single crystal film in the strict sense of the words is not formed on the single crystal substrate, thus forming fine crystals.